Stability, binding, and charge localization in amorphous tin monoxide: a-SnO and a-Sn–Ta–O with variable composition

The structural, electronic, and optical properties of amorphous SnO and Sn–Ta–O (with Sn:Ta ratios 5:1, 3:2, and 1:2) are investigated as a function of density and metal composition using ab initio molecular dynamics simulations and hybrid functional calculations. The short- and medium-range structures of amorphous oxides are thoroughly studied by calculating the effective coordination numbers, effective distances, distortion, and their time variances for metal–oxygen (M–O), O–M, M–M shells, the M–O–M and O–M–O angle distribution, as well as the volume, shape, and distribution of structural voids. Disorder is found to suppress both Sn–O and Sn–Sn coordination but not the O–Sn coordination. Importantly, the threefold coordination of Sn atoms, having a characteristic pyramidal geometry with apical Sn and three oxygen atoms in the base of the pyramid, is maintained within wide ranges of density and metal composition. Weak binding between the chains of corner-shared Sn–O pyramids leads to nearly flat energy–density dependence, suggesting that large density fluctuations are likely to occur in a-SnO and a-Ta2Sn10O15 samples. At an Sn:Ta ratio of 3:2 (a-Ta2Sn3O8), a stable amorphous structure with an optimal density of 6.2g/cm3 provides a large optical bandgap of 2.8 eV combined with the lowest localization of states near the top of the valence band, hence a minimal carrier (hole) scattering.